1. Field of the Invention
The present invention relates to a drive system for a motor vehicle comprising a drive shaft with a flywheel mass arrangement provided thereon and an electric machine connected to the shaft for rotating the drive shaft and/or for obtaining electric energy from the drive shaft, the electric machine having a stator arrangement and a rotor arrangement for rotation with the drive shaft.
2. Description of the Related Art
German reference DE 196 29 346 discloses a hybrid drive system which comprises an internal combustion engine, an automatically operable clutch, an electric motor and a transmission, which are connected in series. A rotor of the electric motor is connected in a rotationally secure fashion to a transmission input shaft of the transmission with insulation regions arranged between the electric motor and the friction clutch and/or the internal combustion engine to prevent the transfer of heat to the electric motor from these heat sources, i.e., the friction clutch and the internal combustion engine. The heat from these heat sources can impair the functionality of the electric motor. In the drive system disclosed in DE 196 29 346 A1, heating of th electric motor from these heat sources is avoided by the relatively large spatial separation between the heat sources and the electric rotor.
Another drive system is disclosed in German reference DE 44 23 577 A1 in which a disk-type flywheel is supported on a drive shaft. A radially outer region of the flywheel forms the rotor of an electric machine that is capable of exerting a torque on the drive shaft to damp torsional vibrations in the drive system. A layer of rubber material is arranged between the radially outward region of the flywheel mass and the radially inward region of the flywheel mass to provide a further vibration-damping function. No statements are made in this reference concerning the thermal properties of the various materials and particularly the components of the drive system, with the aid of which heat could be transferred to the electric machine are not arranged in the region of the electric machine.
It is an object of the present invention to provide a drive system for a motor vehicle comprising an electric machine in which the transfer of thermal energy between the electric machine and a flywheel mass arrangement is decreased to avoid impairing the functionality of the electric machine.
According to the invention, the object is achieved by a drive system for a motor vehicle comprising a drive shaft with a flywheel mass arrangement provided thereon, an electric machine connected to the drive shaft for exerting a torque for rotating the drive shaft and/or for obtaining electric energy from the drive shaft. The electric machine has a stator arrangement and a rotor arrangement connected to the drive shaft via the flywheel mass arrangement so that the rotor arrangement may be rotated by the drive shaft.
In the drive system according to the present invention, the rotor arrangement is connected to the flywheel mass arrangement via a connecting region which is a region having a higher resistance to thermal conduction than at least one adjoining region.
This connecting region may be arranged for effecting a far reaching thermal decoupling of the rotor arrangement and the flywheel mass arrangement. The result of this decoupling is that, for example, there is no risk of impairing the functionality of the entire electric machine by heat generated in the region of the flywheel mass arrangement and transferred to the rotor arrangement.
The increased resistance to thermal conduction may, for example, be provided by construction of the connecting region with an effective cross-sectional transfer area which is diminished with regard to at least one adjoining region.
In this case, the effective cross-sectional transfer area is preferably diminished at least with reference to that adjoining region which is situated on the side on which the increased temperatures are present or are expected to be present.
The reduction of the effective cross-sectional transfer area may be achieved, for example, by including at least one cutout section in the connecting region and preferably a plurality of cutouts. In a preferred embodiment, the cutouts are bores that completely penetrate through the connecting region to amplify the thermal decoupling effect.
At least one cutout may be filled with a material which has a lower thermal conductivity than the material from which the connecting region is formed. The presence of such a material in the at least one cutout has the advantage that no foreign bodies can enter the interior of the rotor through this cutout and be able to cause faults there. Accordingly, the accumulation of dirt or other foreign bodies is thereby prevented in the region of the cutouts.
As an alternative, the connecting region may be formed from a material which has a lower thermal conductivity than the material from which the rotor arrangement is formed and/or the material from which the flywheel mass arrangement is formed.
To additionally remove heat generated in the region of the electric machine and thereby prevent a build up of heat in this region, particularly during an overload condition, the electric machine is arranged so that the rotor surrounds the outer radial side of the stator and at least one cooling rib is provided on the radial outer surface of the rotor.
In general, the electric machine is surrounded on the outside by a fixed housing for purposes of physical protection. In this case, it is further proposed that at least one cooling rib is provided on an inner surface of the housing. In this way, the housing has an enhanced ability to absorb the heat radiated by the electric machine.
The configuration according to the present invention of the drive system is advantageous, in particular, when the flywheel mass arrangement is a temporary heat source. That is, the present invention ensures that the impairment of the functionality of the electric machine does not occur when heat is generated in the region of the flywheel mass arrangement.
For example, the flywheel mass arrangement may comprise a flywheel of a motor vehicle friction clutch. Alternatively, the flywheel mass arrangement may comprise a part of a torque converter such, for example, as its housing.
The electric machine used in the drive system according to the present invention may, for example, comprise a starter/generator machine. This type of device is used both to start an internal combustion engine and to obtain electric energy and, if appropriate, feed the electric energy obtained into a vehicle electrical network whenever the internal combustion engine is running.
The rotor may comprise a rotor excited by permanent magnet.
The electric machine may comprise a three-phase asynchronous machine or a three-phase synchronous machine and may be excited electrically or by permanent magnet. In the case of asynchronous machines, the laminated core provided in the rotor has a relatively high thermal storage capacity and may absorb a portion of the heat which is generated during overload operation of the electric machine. However, there is a risk that when the flywheel mass arrangement is a flywheel of a clutch, for example during engagement of the clutch, there will be an at least short-term excessive rise in temperature with a corresponding impairment of the overload characteristic of the electric machine. This scenario also applies when the electric machine comprises a synchronous machine.
A drive system according to an embodiment of the present invention may comprise an electric machine with a rotor arrangement having a support element which is connected or is connectable in a rotationally secure fashion to the flywheel mass arrangement. In this embodiment, the support element preferably forms the region having a higher resistance to thermal conduction, i.e., the connecting region.
The flywheel mass arrangement may be arranged with external teeth for engaging the support element. These external teeth may simultaneously be used with an electromagnetic pickup to detect the speed of the flywheel mass arrangement, which corresponds in general to the speed of a drive unit.
The support element may comprise a plurality of engaging projections which project from a basic body for producing the rotary connection to the flywheel mass.
Alternatively, the support element may be connected, or be connectable, to the flywheel mass arrangement by a plurality of fastening bolts. To obtain a very effective thermal insulation here, a bearing surface segment of one of the flywheel mass arrangement and the support element has a smaller surface area than a bearing surface segment on the other one of the flywheel mass arrangement and the support element. For example, one bearing surface segment may form at least one bearing surface of essentially annular construction.
In a further alternative embodiment of the drive system according to the present invention, the rotor arrangement comprises a plurality of metal disk elements which are of essentially annular construction and are supported by a plurality of fastening bolts on the flywheel mass arrangement. In this embodiment, the fastening bolts preferably essentially form the region having a higher resistance to thermal conduction, i.e., the connecting region.
In this embodiment, a desired spacing between the metal disk elements, which here form a yoke for the rotor, and the flywheel mass arrangement, is effected by providing at least one spacer element between the flywheel mass arrangement and the metal disk elements. The at least one spacer element provides a higher resistance to thermal conduction with regard to the flywheel mass arrangement and/or the fastening bolts.
To further support the removal of heat in the drive system according to the present invention, a coolant duct may be arranged in a fixed portion of the drive system, i.e., a portion of the drive system which does not rotate with the rotor arrangement.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.